1. Field of the Invention
The present invention relates to a drive circuit and more particularly to a laser array circuit in which charges are sequentially accumulated in a charge capacitor associated with a laser diode array scheduled to emit light by repeatedly switching a laser diode driving element on and off in order to cause multiple laser diodes to sequentially emit light so as to generate pulsed light.
2. Description of the Related Art
A conventional semiconductor laser beam scanner is proposed, for example, in JP-A-1-152683. Specifically therein, a semiconductor laser beam scanner is described including a semiconductor laser array having multiple laser diodes in alignment, and a convex lens disposed in a laser-beam emitting direction of the semiconductor laser array.
The semiconductor laser beam scanner adopts an electronic scanning method that includes deflecting a laser beam at an angle determined based on the position of a light-emitting point of the semiconductor laser array and the focal length of the convex lens. The laser diodes are sequentially lit by drive circuits, which are individually connected to the respective laser diodes, while having a time difference created by the drive circuits. Since a range that can be scanned by the laser diodes is determined when a laser beam is sequentially emitted from the semiconductor laser array in a direction that begins with one end of the semiconductor laser array, an entity located in front of the semiconductor laser beam scanner is scanned.
In general, for a drive circuit that drives a sole laser diode, a method can be adopted involving applying a steeply pulsating current to a laser diode on the basis of charge accumulated in a capacitor by turning on a switching element such as a MOSFET. In such a method, a current of several tens of amperes must be applied to each of the laser diodes for a period of several tens of nanoseconds in order to produce the required pulsed light. Therefore, a MOSFET having the ability to switch conduction of a large current and non-conduction thereof at a high speed with a low on resistance must be selected. A chip size associated with the MOSFET having such an ability is large, on the order of, for example, 3 mm by 3 mm. A circuit pattern having interconnected laser diodes, capacitors, and MOSFETs must be wired with thick and short connections.
However, the adoption of the MOSFET of the foregoing size makes it hard to minimize the size of the circuit pattern. Referring to FIG. 10, a description will be made of a reason why the adoption makes the minimization hard.
FIG. 10 shows a layout of a semiconductor laser array 30, MOSFETs 31, chip capacitors 32 on a circuit board 33. The chip capacitors 32 and MOSFETs 31 are laid out on the circuit board 32 in association with laser diodes in order to individually drive the laser diodes so as to sweep a laser beam.
In such a case, the chip capacitors 32 and MOSFETs 31 are laid out with the lengths of wirings equalized so that the wiring impedances each including a parasitic resistance and a parasitic inductance produced on each of the wirings laid to link the laser diodes and chip capacitors 32 will square with one another. However, as shown in FIG. 10, when the MOSFETs 31 of the foregoing size are arrayed on the circuit board 33, the number of MOSFETs 31 that can be disposed on the circuit board 33 is limited. Eventually, the number of laser diodes is limited. Therefore, a wide range cannot be scanned with a high degree of precision.
Increasing the number of laser diodes requires a corresponding increase in the number of MOSFETs 31, leading to a disadvantageous increase in the size of the circuit board 33. As the circuit pattern gets larger, the wiring impedance including parasitic resistance and parasitic inductance gets larger and may give rise to propagation conditions where a pulse width may be expanded, a pulse may be split into multiple parts, or a pulse may be lost. Further, the waveform of a pulse may be destroyed, or a crest value may be decreased and it becomes difficult or impossible to feed the driving circuits with a steeply pulsating current.